Telegraph transmitter with amplitude-modulation



Sept. 15, '1,952 H. B. R. BoosMAN ET A1. 2,611,078

TELEGRAPH V*'IRANSMIT'IER WITH AMPLITUD-MODULATION Filed June 30, 1948 DUTPUT H-F AMPLlFlER O5 CILLATOR 1@ FL MovuLM'oszJ THREsHoLD Device OSCI LLATOR OUTPUT TUBE i 5 'y' OSCILLAIOR OUTPUT@ 57.6 3 s1' T RELAY Patented Sept.

TELEGRAPH TRANsMirfrEn 'vvirr-iy AMPLITUDE-MODULATION l l dolfv Boosman and Paul Netherlands, assignors to Hartford National Bank and Trust,:('lompannv Herman Bernard Ru Zijlstra; Eindhoven,

l Hartford, Conn.,.as trustee 1948, serial No.

Application June. 30, y

dsJime 3, 191WY In' tliaNetherlan claims. (ci. '25o-8) The invention relates to a radio transmitter for transmitting a carrier oscillation modulated in amplitude by telegraphc signals, for example, Morse signals.

Such a telegraph transmitterwith amplitudemodulation requires to be arranged so as to prevent detrimental overload of the transmitter even in the case of abnormal manipulation of the sending key, but'thus in normal use transmission is not carried out with maximum transmitting power. y

' The invention has forv its object to provide a form of telegraph transmitter in which transmission is effected with maximum power in normal' use and abnormal operation conditionsinvolve neither harmful overload'n'or interruption of operation. n Y Y According to the invention, for vthis purpose a measuring value is produced `which varieswith the temperature of an outputvalve of the transmitter and means `are provided to limit the transmitter load for at least the period `over which the said measuring value'exceeds a denite limit value which corresponds tol'avtemperature lower than the maximum permissible temperaf ture oi the said output valve.

The use of the inventionA permitsioiarrang ing the transmitter in such manner that in normal use, in which a normal ratio between signal and duration of interval4 (about 1:1)l maybe relied on, the transmitter is practically loaded toits maximum value. In the` case ofv abnormal transmission signalsgforv example' ifl a particularly large number of dashesl is. given with short intervals, or else if in view of the tuningv ofa second station, the sending key is depressedfor 30 seconds or more; restriction of the transmitter load automatically occurs in due time, in order to avoid excessive temperature'of the output valve or valves and hence the necessity of interrupting operation. t

The measuring value required .for automatic load restriction and corresponding to the temperature of the output' valve' ofthe' transmitter may be ascertained directly with the usev of a thermometer checking the temperature of the tube.

However, as an alternative the temperature may be measured indirectly by'supplyinga voltage proportional to the modulating or modulated voltage to an integrating network, oi which the condenser (integration condenser), across Which the integrated value occurs, is shunted b-y a discharge resistance. The condenser voltagethus constitutes a measuring value'varying with the 2; temperature ofuthe output; valve of the transmitter. f 1

A method of bringing about the load restriction of the transmitter consists iny increasing the negative control-grid bias voltage, of the output valveI (s) oi the transmitter.

A further method,.which is greatly preferred, consists inr'educing..thevmodulating voltage in theA rhythm oiarequency of Vwhich one period is' smaller than thev duration of theish'ortest-y signal, which readily permits at the. receiver end of avoiding. spurious reproduction of the, signals. The modulating voltagemay; for'example, be intermittently interrupted.V r A particular advantage of-'the latter method consists vinthat, in spite of the reduction in' load;olf the transmitter the signal-noise ratio of thezitelegraph signals remains unchanged' upon reception, so that in practicev the rangeof theztransmitter is not reducedl Y v In order that the invention'v may` be more clearly understood andreadily. carried into eiect, it will now be described more fullyvvith reference tothe accompanying drawings,` elements known per "sel being shown onthe figures in the' form offblocks and corresponding elementsy being designated by' like reference numerals.

Fig. 1 showsa telegraphltransmitter according to the inventionthe measuring value corresponding to thel temperaturey of the output valve of? the `trainsmitter .beingascertainedi direotly.

Fig. 2 showsa-telegraphftransmitter according to the invention, a mez'ts'uring value correspondk ing to the temperature'o'i the output valveof the transmitter being ascertainedindirectly with the use oi4 an integrating network;

'Figi 3 shows some tevvv diagramsto illustrate the operation of the telegraph transmitter shown in Fig; 2f; y

l.=g".-4y shows a variantv in the telegraphv transmitter shown 1n- Fig'. .2;

Fig; slfiowsV diagrams to illustrate the operation of' the telegraph transmitter shown-in Fi'gl.4; Fig. 6 shows a telegraph transmitter" according tol the invention, a measuring vaille? Corresponde ing to the temperature of the output' valveY oi the transmitter being ascertained indirectly' by supplying the detected, andi modulateof direet current signals to an 'integrating network'.

In the telegraph transmitter shown in"A Fig".- 1 the modulating direct-currer-it-signalsV are ied from asendingkeyland a` battery 2 1 to a modi'll'at'ol* 3, )sl'hicl'iV is-'vcn'nec-ted' tO a high-#frequency oscillator afror'tiie supply of the-fcarier'wave.

3 Subsequent to amplification in a transmitter output ampliiier to the modulated oscillations are supplied to an aerial 6. The temperature of the output valve (valves) of the transmitter is ascertained with the use of a thermometrcal Vdevice 1 and converted into an electrical measuring value. Thermometrical devices of this kind may be constituted, for example, by a radiation thermometer (bolometer or photo-cell) or a thermopile or other electric thermometer which indicates the temperature of the output valve of the transmitter. With forced cooled transmitting tubes the temperature of the cooling agent is a measure of the temperature of the tube, so that it may form the starting point.

In the form shown in Fig. 1 the thermometer voltage is amplied by a direct-current amplifier 8 and then fed to a threshold device 9. If the voltage supplied by the amplifier 8 exceeds a limit value determined by the threshold device 9 and corresponding to a temperature lower `than the maximum permissible temperature of the output valve of the transmitter, an output voltage of the threshold device 9 occurs which is fed, as a control-voltage, to the grid of the output valve(s) of the transmitter in such manner that the dissipation is restricted.

In the embodiment shown in Fig. 2, I9 designates a sending key, which is connected in series with a battery I and a signalling relay I2. The signalling relay I2 comprises a make contact I2', included in the grid circuit of a modulator I4, which normally is cut oi by a suitable bias Voltage, is constituted by the output valve of the transmitter and is connected to a carrier-wave oscillator I6. The modulated oscillations are emitted by an aerialrI5.

On closing of the sendingkey I9 the relay I2 is energized and the make contact I2 is closed. The cut-off bias voltage of the modulator I4 is thus rendered inoperative by the positive auxiliary voltage set up by a battery I3 in the grid circuit of the modulator valve. A

In contrast with Fig. 1, Fig. 2 shows indirect temperature measurement, i. e. by supplying the modulating signals occurring across the series combination of the sending key IU' and the battery II to an integrating network comprising a resistance I'I and an integrating condenser I8. The voltage across the integrating condenser I8 controls a maximum relay I9 connected in parallel therewith, which on response brings about a restriction of the transmitter load in a manner to be described hereinafter.

The signals supplied to the integrating network I1, I8, I9 bring about a charge of the integrating condenser I8, which charge depends on the time constant of the charging circuit of the condenser I 8. which is determined by the capacity of the latter, the resistance Il and the resistance of the maximum relay I9. When the sending key I9 is open, the condenser I8 is discharged through the maximum relay I9. The discharge circuit connected in parallel therewith, which comprises the series-combination of the resistance I1 and the resistance of the signalling relay I 2, is negligible in practice.

If the charging and discharging circuits of the condenser I8 are correctly proportioned, the voltage across the integrating condenser I8 constitutes a measuring value corresponding to. the temperature of the output valve of the transmitter. In practice it is preferred to provide that during a signal the integration voltage increases more rapidly than the temperature of the output 4 valve of the transmitter and this may be achieved in a simple manner by choosing the time constant of the charging circuit of the integrating condenser IS to be smaller than the time constant for the increase in temperature of the loaded output valve of the transmitter. The discharge circuit of the integrating condenser I8 is preferably chosen to be such that its time constant approximately corresponds with the time constant for the decrease in temperature of the unloaded output valve of the transmitter.

In normal operation, when transmission takes place with maximum output and the output valve of the transmitter reaches a denite limit value corresponding to a temperature lower than the maximum permissible temperature, the voltage of the integrating condenser I8 will not be increased to the value of energisation of the maximum relay I9. In the case of abnormal manipulation of the sending key I9, for example, when the-sending key remains closed for a long period, the temperature of the output valve of the transmitter will exceeed the said limit value and the energisation of the maximum relay I9 results in due time in such a restriction of the transmitter load that the maximum permissible temperature of the output valve of the transmitter is not exceeded.

In order to follow the temperature variation of the tube in the measuring circuit I'I, I8, I9 in the case of load restriction of the output valve of the transmitter, the measuring circuit constants also require to be modified. For this purpose the maximum relay I9 of Fig. 2 comprises a make contact I9., which in 'its closed position connects a supplementary discharge resistance 20 in parallel with the integrating condenser I8.

The load restriction is achieved as follows: on the maximum relay I9 being energized, an alternating contact I9 of the maximum relay I9 connects an oscillator 2| which produces a sinusoidal voltage into the grid circuit of the output valve of the Ytransmitter and also a battery 22, which supplies a supplementary positive bias voltage which is substituted for the higher positive bias voltage of the battery I3, which voltage is now cut oif. 'I'he value of the battery and oscillator voltage is chosen to be such that, with continuous signal, the temperature of the output valve of the transmitter is kept below the maximum permissible value owing to the decrease of the modulating signal voltage, which decrease periodically occurs as a result of the sinusoidall voltage. In order to prevent signals from failing to appear due to the additional modulation, the frequency of the sinusoidal modulation must be chosen to be so high that one period thereof is small as compared with the duration of the shortest signal.

Instead of using a sinusoidal voltage for load restriction, use may be made of a triangular or rectangular voltage. Thus, for example, the modulating voltage may be intermittently interrupted with the use of an interruptor, for example, a multivibrator circuit.

Fig. 3 shows a diagram to illustrate the operation of the device shown in Fig. 2.

In the flgure the variations of the voltage across the condenser I8 and the temperature of the output valve I4 of the transmitter are plotted against the time t, if the transmitter is loaded at the instant t=0 by closing the sending key Iii. The temperature and voltage variations are shown by curves a` and b respectively. At the instant t=t1 the temperature of tube I4 reaches the predetermnedlimt value. whilstat the same lperiodic interruption of the sarees ceeds the response. voltage c of' themaximum rlel'aylv i9, 'which brings about. a reduction. in load of the `output valve. off thetransmitter andfthus keepstlf'i'etemperatu'rer of tubeibelow themaximum value. permissible'or. this tube.

Fig. shows a telegraphr transmitter according to .the invention, in wlii'cli.the signalling relay alsol actsY as'` van.I interruptor.. A. sendingjkey 2?. is connectedv inv serieswith a signalling. relay 24', aAv battery-K25;` and an` integrating. condenser 261 The signalling.V- relay. comprises anV alternating contact .24! inE its. positionh ofrrest), the

lower contactgpointfoflwhich isconnectedito the control-grid ofi-the''outputvalvey liti-cfv the-transmitter, whereas the other contant''point is connected, through a resistance 21, in seriesV with the sending key 231.. signalling:relay 24 and battery 25. On` the sending key 23 being closed, the signalling relay 24 is' energized so that' the integrating condenser 26 is charged through the resistance of the signalling relay. thesending key being opened, the signalling" relay becomesdeenergized,y which permits oi theintegrating condenser -25- being discharged through the: resistance 21. AThe charge and discharge circuits being suitably fproportioned'-,V the' voltage across` the integrating condenser 26-constitutes a measuring value varying with the temperature of the output valve of the transmitter. If the condenser voltage exceeds a rst threshold value which corresponds to the limit value which obtains for the temperature of the output valve of the transmitter, the relay 24 is de-energized with the result that the modulating voltage of the output valve I4 of the transmitter is interrupted. The integrating condenser thus becomes discharged through the parallel combination of the resistance 21 and the resistance of the signalling relay 24. If, however, the voltage of the condenser 26 falls below a second threshold value, the signalling relay is again energized, and so forth. Thus sending current is produced and the circuit constants being suitably chosen a normal signal-interval ratio of, for example, 1:1 is again attained in spite of the sending key being kept closed continuously, so that excessive load of the output valve of the transmitter is avoided.

The interruption frequency may be chosen to be so high that sending-current meters or indicators of standard type used at the receiver end do not give unsteady indication. If such a choice of the interruption frequency is not readily possible, for example from considerations of the available bandwidth, the interruption frequency may be temporarily increased, particularly in view of the tuning of a receiver co-operating with the transmitter, or else the dissipation in the output valve of the transmitter may be reduced to a permissible value by variation of the grid bias Voltage.

Fig. 5 shows some few time diagrams to illustrate the operation of the telegraph transmitter shown in Fig. 4.

Fig. 5a illustrates a number of direct-current pulses which form the modulating voltage for the output valve of the transmitter. The signalinterval ratio of these direct-current pulses is deliberately chosen to be higher than 1, so that the temperature of the output valve of the transmitter will exceed the limit value obtaining therefor, which results in load restriction.

Fig. 5b illustrates the voltage across the integrating condenser 26 due to the signals shown in Fig. 5a. Boththreshold values'i'. e. those values of' the voltage across the condenser 2'6` at which the maximumv relay'. 2l'. becomesY energized. and deenergized correspond to" the 'two vlinesi Si and S'z, which are drawn parallelY to the time axis; With given circuit constants, the interruption frequency' is determined principally by the diiierence between the threshold'. values.

' Fig. 5c shows the variation ofthe modulating voltage ofI the output valve'. of the'transmitter' Il. Asrrnay be seen from the ligure the signalsbetween the instants Ol-futcv-'tati-ts areY transmitted in a normalmanner,l whereas the. signals between tir-f7 and 'tt-tgare intermittently interruptedl In the telegraphV transmitter shown in Fig'. 6' theA modulating direct-current signals from'. a sending key landla battery.` '2L'are1' fedto a transmitter output Tand modulator valve H, which is also 'connectedto the carrier-wave, oscillator i6. Part oi'.` the output energy'oflthe. output valve M of the Vtransmitter,- is detecte'dLby a re'ctier v28 and' then supplied to a temperature-indicating device comprising a resistance 29., a condenser 3l] and a maximum relay 31- 'If' the voltage across the condenser 30 exceeds aiirst threshold value. which corresponds to alimit' value'obtainingl for the. tei'nperature of Vthe.v said" output-valve lll of the transmitter, the relay 3'i`fi'sjenergi'zed', closes a make contact 3l', which results' in cutting oi the output valve I4 of the transmitter with the use of a battery I3. The condenser 30 is thus adapted to discharge through the resistance of the maximum relay 3|. If, however, the voltage across the condenser 30 falls below a second threshold value, the relay 3l 'is de-energized and so forth, so that intermittent interruption of the signals re-occurs.

What we claim is:

1. In a radio-telegraph transmitter constituted by a radio-frequency power amplifier provided with an electron discharge output tube, means coupled to said amplier to adjust the average power consumed thereby, and a modulator actuated by a telegraph key to render said amplifier operative accordingly, the power consumed by said amplifier and the attendant temperature rise of said output tube depending on the time within a given period during which said key is closed: apparatus for preventing a temperature rise in said output tube beyond its maximum permissible value without interference to telegraph transmission, said apparatus comprising means to develop a control voltage whose amplitude varies in accordance with the temperature of said tube. and means responsive to amplitudes of control voltage exceeding that amplitude corresponding to said maximum permissible value and coupled to said power adjusting means to limit the average power consumed by said ampliiier during telegraph modulation to a level at which the temperature of said tube is at a permissible value.

2. Apparatus as set forth in claim 1 whereinl including a telegraph key connected in series with a direct-voltage source, the power consumed by said amplifier and the attendant temperature rise of said output tube depending on the time within a given period during which said key is closed: apparatus for preventing a temperature rise in said output tube beyond its maximum permissible value without interference to transmission, said apparatus comprising an integrating network dened by a capacitor in series with a resistance, means toconnect said network across said serially-connected key and voltage source, said network having a time constant at which the voltage developed across said capacitor has an amplitude which varies in accordance with the temperature of said tube, and means responsive to an amplitude of voltage developed across said capacitor exceeding that amplitude corresponding to said maximum permissible value to periodically interrupt the actuation of said modulator thereby to limit the average power consumed by said amplier to a level at which the temperature of said tube is at a permissible value.

4. An arrangement, as set forth in claim 3, wherein said means responsive to an amplitude of voltage exceeding that corresponding to said maximum permissible value to periodically interrupt the actuation of said modulator includes a low-frequency oscillation generator, an electromagnetic relay connected across said capacitor. said relay being energized when the amplitude of voltage developed across said capacitor exceeds the amplitude corresponding to said maximum permissible value, said relay being arranged to connect said generator to the input of said modulator to impose" the low-frequency oscillations as a modulationj component on said telegraph signals whereby the actuation of said modulator is periodically interrupted at a rate corresponding to the frequency of said oscillations.

5. Anarrangement, as set forth in claim 4, further including a discharge resistance, and means to connect said discharge resistance across said capacitor upon said being energized.

HERMAN BERNARD RUDOLF BOOSMAN.

.PAUL ZIJLSTRA.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

